Methane Hydrates in Nature - Current Knowledge and Challenges
Collett, T., Bahk, J. J., Baker, R., Boswell, R., Divins, D., Frye, M., ... & Torres, M.
(2015). Methane Hydrates in Nature - Current Knowledge and Challenges. Journal
of Chemical & Engineering Data, 60(2), 319-329. doi:10.1021/je500604h
10.1021/je500604h
American Chemical Society
Version of Record
http://cdss.library.oregonstate.edu/sa-termsofuse
Article
pubs.acs.org/jced
Methane Hydrates in NatureCurrent Knowledge and Challenges
Tim Collett,*,† Jang-Jun Bahk,‡ Rick Baker,§ Ray Boswell,§ David Divins,∥ Matt Frye,⊥ Dave Goldberg,#
Jarle Husebø,∇ Carolyn Koh,¶ Mitch Malone,● Margo Morell,∥ Greg Myers,∥ Craig Shipp,□ and Marta Torres▲
†
U.S. Geological Survey, West 6th Ave. & Kipling St., Lakewood, Colorado 80225, United States
Korea Institute of Geoscience and Mineral Resources, Yuseong-gu, Daejeon, 305-350, Korea
§
U.S. Department of Energy, National Energy Technology Laboratory, 3610 Collins Ferry Road, Morgantown, West Virginia 26507, United States
∥
Consortium for Ocean Leadership, Washington, DC 20005, United States
⊥
U.S. Bureau of Ocean Energy Management, Herndon, Virginia 20170, United States
#
Lamont-Doherty Earth Observatory, 61 Rte 9w, Palisades, New York 10964, United States
∇
Statoil ASA, Forusbeen 50, 4035 Stavanger, Norway
¶
Colorado School of Mines, 1500 Illinois St, Golden, Colorado 80401, United States
●
Texas A&M University, College Station, Texas 77843, United States
□
Shell International Exploration and Production Inc., 200 N Dairy Ashford Rd, Houston, Texas 77079, United States
▲
Oregon State University, Corvallis, Oregon 97331, United States
‡
ABSTRACT: Recognizing the importance of methane hydrate
research and the need for a coordinated effort, the United States
Congress enacted the Methane Hydrate Research and Development Act of 2000. At the same time, the Ministry of International
Trade and Industry in Japan launched a research program to
develop plans for a methane hydrate exploratory drilling project in
the Nankai Trough. India, China, the Republic of Korea, and other
nations also have established large methane hydrate research and
development programs. Government-funded scientific research
drilling expeditions and production test studies have provided a
wealth of information on the occurrence of methane hydrates in
nature. Numerous studies have shown that the amount of gas
stored as methane hydrates in the world may exceed the volume of
known organic carbon sources. However, methane hydrates represent both a scientific and technical challenge, and much remains to be
learned about their characteristics and occurrence in nature. Methane hydrate research in recent years has mostly focused on: (1)
documenting the geologic parameters that control the occurrence and stability of methane hydrates in nature, (2) assessing the volume of
natural gas stored within various methane hydrate accumulations, (3) analyzing the production response and characteristics of methane
hydrates, (4) identifying and predicting natural and induced environmental and climate impacts of natural methane hydrates, (5) analyzing
the methane hydrate role as a geohazard, (6) establishing the means to detect and characterize methane hydrate accumulations using
geologic and geophysical data, and (7) establishing the thermodynamic phase equilibrium properties of methane hydrates as a function of
temperature, pressure, and gas composition. The U.S. Department of Energy (DOE) and the Consortium for Ocean Leadership (COL)
combined their efforts in 2012 to assess the contributions that scientific drilling has made and could continue to make to advance our
understanding of methane hydrates in nature. COL assembled a Methane Hydrate Project Science Team with members from academia,
industry, and government. This Science Team worked with COL and DOE to develop and host the Methane Hydrate Community
Workshop, which surveyed a substantial cross section of the methane hydrate research community for input on the most important
research developments in our understanding of methane hydrates in nature and their potential role as an energy resource, a geohazard,
and/or as an agent of global climate change. Our understanding of how methane hydrates occur in nature is still growing and evolving,
and it is known with certainty that field, laboratory, and modeling studies have contributed greatly to our understanding of hydrates in
nature and will continue to be a critical source of the information needed to advance our understanding of methane hydrates.
Special Issue: In Honor of E. Dendy Sloan on the Occasion of His
70th Birthday
Received: June 30, 2014
Accepted: September 22, 2014
Published: October 7, 2014
© 2014 American Chemical Society
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1. INTRODUCTION
In 2012, the U.S. Department of Energy’s (DOE) National
Energy Technology Laboratory (NETL), in partnership with the
Consortium for Ocean Leadership (COL), initiated a new fieldfocused methane hydrate research project that would inform, and
potentially lead to, future offshore drilling expeditions. The
primary objective of this project was to identify how scientific
drilling could be most effectively conducted to advance the
evaluation of methane hydrate resource, geohazard, and climatechange implications. To implement and help guide this effort,
COL assembled a Methane Hydrate Project Science Team
consisting of representatives from academia, industry, and
government. Two of the major elements of this COL-led science
planning effort were (1) the authoring of a Historical Methane
Hydrate Project Review Report1 and (2) the hosting of a
Methane Hydrate Community Workshop.2 The historical review
report was used as a guide to develop the agenda for the Methane
Hydrate Community Workshop and provide the foundation for
the Methane Hydrate Research Science Plan.3
The COL-hosted Methane Hydrate Community Workshop
focused on identifying and assessing specific scientific challenges
that must be addressed to advance our understanding of methane
hydrates and how these challenges could be resolved with
the support of scientific drilling. The workshop also provided an
excellent venue for the exchange of ideas among a highly
interdisciplinary group of scientists. Workshop discussions
provided detailed reviews of our current understanding of the
geologic controls on the occurrence of methane hydrate in nature
and how these factors may impact the energy, hazard, and climate
change aspects of methane hydrate research. It was concluded
that the most significant advancements in methane hydrate
research have included:
-Documenting the geologic parameters that control the
occurrence and stability of hydrates in natureMethane
Hydrate System;
-Assessing the volume of natural gas stored as hydrates within
various geologic settingsMethane Hydrate Assessments;
-Analyzing the production response and characteristics of
methane hydratesMethane Hydrate Production;
-Identifying and predicting natural and induced environmental
and climate impacts of natural methane hydratesMethane
Hydrate Climate Change Issues;
-Analyzing the impact and response of methane hydrates to
external forcingMethane Hydrate Geohazard Issues;
-Development of advanced methane hydrate modeling,
laboratory and field characterization capabilitiesPressure
Core Field and Laboratory Studies, Downhole Measurements
Systems, Reservoir Modeling, Drill Site Geologic and Geophysical Characterization.
In this report we summarize our most current understanding
of methane hydrates in nature and further document the
technical and scientific challenges that will be the likely focus of
methane hydrate research in the next decade.
study of methane hydrates in nature and explore future methane
hydrate research opportunities.
2A. Methane Hydrate System. In recent years, significant
progress has been made in addressing key issues on the formation, occurrence, and stability of methane hydrate in nature.
The concept of a methane hydrate system, similar to the concept that guides conventional oil and gas exploration, has been
developed to systematically assess the geologic controls on the
occurrence of methane hydrate in nature (Figure 3).6,7 In a
methane hydrate system, the individual factors that contribute
to the formation of methane hydrate in nature can be identified
and assessed; the most important factors include: (1) methane
hydrate pressure−temperature stability conditions, (2) gas
chargethe combination of gas source and migration, and (3)
the presence of suitable host sediment or “reservoir”.
In terms of methane hydrate as a potential energy resource, the
concept of a methane hydrate system has been developed to
systematically assess the occurrence, distribution, and richness of
gas hydrate accumulations in nature from the reservoir to a basin
scale.7 The methane hydrate system concept has been used to
guide the site selection process for numerous recent national and
international methane hydrate scientific drilling programs.8 The
methane hydrate system concept can also be used to characterize
the geologic controls on the occurrence and stability of methane
hydrates in natural systems with respect to geohazard and
climate-change response assessments.
Most methane hydrate system studies have focused on
describing hydrates as static deposits rather than building a
better appreciation of them as part of a dynamic system. Fundamental questions remain as to the residence time of methane
hydrates near the seafloor and deeper within the sediment
column, the sources of methane and the pathways for its
transport, the nature and mechanisms driving fluid flow, and
changes in these variables through time (Figure 3). Consequently, there is a growing imperative to develop integrated
time-dependent models to understand the controls on the
formation, occurrence, and stability of methane hydrates in
nature, as well as the forcing mechanisms that modulate the
processes responsible for methane generation, consumption, and
potential discharge from the methane hydrate system.
2B. Methane Hydrate Assessments. Methane hydrate
resource assessments indicating enormous global volumes of
methane within hydrate accumulations have been one of the
primary driving forces behind the growing interest in methane
hydrates.9 For the most part, these estimates range over several
orders of magnitude, creating great uncertainty in the role
methane hydrates may play as an energy resource or as a factor in
global climate change. In recent years, field production tests
combined with advanced numerical simulation have shown that
hydrates in sand reservoirs are the most feasible initial targets for
energy recovery. It has also been shown that, with regard to the
climate implications of methane hydrates, there is growing need
to accurately assess the portion of the global methane hydrate
endowment that is most prone to disturbance under future
warming scenarios.
Over the last 10 years, a number of new quantitative estimates
of in-place methane hydrate volumes have been reported.10−13 In
most cases, hydrate assessments include the analysis of a set
of minimum source-rock criteria such as organic richness,
sediment thickness, and methane generation as they apply to
both microbial and thermogenic gas sources (Figure 4). In
several of the more recent assessments, the hydrate resource
volume estimates have also considered the nature of the
2. STATE OF METHANE HYDRATE SCIENCE
The study of methane hydrates in nature has been ongoing
for over 40 years. Significant strides have been made in our
understanding of the occurrence, distribution, and characteristics
of marine methane hydrates (Figures 1 and 2), but knowledge of
the role they may play as an energy resource, geologic hazard, and
possible agent in climate change is incomplete. In this section, we
review some of the most important recent developments in the
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Figure 1. Location of sampled and inferred methane hydrate occurrences in oceanic sediment of outer continental margins and permafrost regions.4
Most of the recovered methane hydrate samples have been obtained during deep coring projects or shallow seabed coring operations. Most of the
inferred methane hydrate occurrences are sites at which bottom simulating reflectors (BSRs) have been observed on available seismic profiles. The
methane hydrate research drilling projects and expeditions reviewed in this report have also been highlighted on this map.
Figure 2. Timeline chart showing the deepwater marine, Arctic permafrost, and academic ocean drilling scientific drilling expeditions dedicated to the
research on naturally occurring methane hydrates.5
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Figure 3. A schematic depiction of the components of various types of methane hydrate systems.6 Typical methane hydrate reservoir morphologies
including (A) networks of hydrate-filled veins; (B) massive hydrate lenses; (C) grain-filling methane hydrate in marine sands; (D) massive sea-floor
mounds; (E) grain-filling methane hydrate in marine clays; (F) grain-filling methane hydrate in onshore Arctic sands/conglomerates. The general
location of the most resource relevant (blue circles) and most climate relevant (green circles) methane hydrate occurrences are also shown. Images as
shown are (A) courtesy UBGH-01 (Korea); (B) courtesy NGHP Expedition 01 (India); (E) courtesy GMGS Expedition 1 (China); (F) courtesy Mallik
2002 Science Program (Canada). Other parts of the methane hydrate system as depicted include the relationship between microbial and thermogenic
gas sources and gas migration controls.
Figure 4. Map of the methane hydrate stability zone thickness used to limit the area assessed for the occurrence of methane hydrate within the
worldwide methane hydrate assessment conducted by Wood and Jung.12
sediments that host the hydrates. For example, in 2012, the
Bureau of Ocean Energy Management (BOEM) reported that
the “United States Lower-48 Outer Continental Shelf” contains
about 1454 trillion cubic meters (51 338 trillion cubic feet)
of in-place gas within hydrates. BOEM further estimated that
190 trillion cubic meters (6700 trillion cubic feet) of the total
amount of gas within methane hydrates of the Gulf of Mexico
occurred as highly concentrated hydrate accumulations within
sand reservoirs. Similarly, BOEM estimated the volume of gas
within sand reservoirs along the Atlantic margin of the United
States at 447 trillion cubic meters (15 785 trillion cubic
feet). Fujii et al.14 also reported on a resource assessment of
methane hydrates in which they estimated the volume of gas
within the hydrates of the eastern Nankai Trough at about
1.1 trillion cubic meters (40 trillion cubic feet), with about half
of the estimated volume concentrated in sand reservoirs.
Collectively, the recent hydrate assessments in the United
States and Japan indicate that reservoir-quality sands may be
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more common in methane hydrate systems than previously
thought.
One of the most important emerging goals of methane hydrate
research and development activities is the identification and
quantification of the amount of technically and economically
recoverable natural gas that might be stored within methane
hydrate accumulations. In 2008, the U.S. Geological Survey used
the methane hydrate system concept for the first time to assess
amount of the technically recoverable methane hydrate resources
on the North Slope of Alaska;15 this assessment indicated that
there are about 2.42 trillion cubic meters (85.4 trillion cubic
feet) of technically recoverable methane gas resources within
concentrated, sand-dominated, methane hydrate accumulations
in northern Alaska.
A variety of models have been developed to predict methane
hydrate occurrence on local, regional, and global scales. But to
properly constrain predictive assessment models, it is critical
to have a comprehensive understanding of the model input
parameters, in particular, variables that control the input and
output of methane over time. Additionally, while sensitivity
studies can identify the most important components in any one
model, it is often uncertain which of the many critical parameters and conditions are the primary driving forces for the
accumulation of methane hydrate in the natural environment at
any specific site.
2C. Methane Hydrate Production. Methane hydrates are
known to occur at high concentrations in sand-dominated
reservoirs in both permafrost16 and deep marine settings.8,17
These settings have been the focus of recent methane hydrate
exploration and production studies in northern Alaska and
Canada, in the Gulf of Mexico, off the southeastern coast of
Japan, in the Ulleung Basin off the east coast of the Korean
Peninsula, and along the eastern margin of India (Figure 1).6
Production testing and modeling have shown that concentrated
methane hydrate occurrences in sand reservoirs are conducive to
existing well-based production technologies (Figure 5).9 Because
conventional production technologies favor sand-dominated
methane hydrate reservoirs, sand reservoirs are considered to be
the most viable economic target for methane hydrate production
and will be the prime focus of most future methane hydrate
exploration and development projects, with depressurization
techniques, optimized through tailored applications of thermal
and/or chemical stimulation, being the primary basis for initial
production experiments.6,18
Over the last 10 years, national methane hydrate research
programs, along with industry interest, have led to the development and execution of major methane hydrate production
field test experiments. Three of the most important field
testing programs have been conducted at the Mallik site in the
Mackenzie River Delta of Canada19,20 and in the Eileen methane
hydrate accumulation (i.e., Mount Elbert and Ignik Sikumi tests)
on the North Slope of Alaska.21 Most recently, we have also seen
the completion of the world’s first marine methane hydrate
production test in the Nankai Trough offshore of Japan.22 The
recent production tests in Alaska, northern Canada, and offshore
Japan have collectively shown that natural gas can be produced
from methane hydrates with existing conventional oil and gas
production technology.
In general, the completed field methane hydrate production
tests have been of limited duration, from 6 to 25 days. These tests
support the technical proof-of-concept for gas production from
hydrate reservoirs, but they fall short of proving the economic
viability of the resource. Longer-duration production tests that
Figure 5. Schematic of proposed methane hydrate production
methods.7
rigorously test a wide range of production technologies are
needed to investigate the viability of economic gas production
from methane hydrate. The need for future methane hydrate
production testing at the experimental and industry-scale to
understand the ultimate economic resource potential of methane
hydrates is critical. To prepare for future field production testing,
more information is needed (1) on the properties/characteristics
of methane hydrate reservoirs, (2) the production response of
various methane hydrate accumulations measured in the
laboratory and quantified through production modeling, and
(3) the environmental issues controlling the ultimate resource
potential of methane hydrates. Numerical models that can
accurately represent observed phenomena in field and laboratory
experiments also need to be further developed and improved.
2D. Methane Hydrates and Climate Change. Methane
hydrates are only recently being incorporated into global
environmental concepts and processes, including carbon
cycling23 and global climate.24 For example, the atmospheric
concentration of methane, like that of carbon dioxide, has
increased over the past century. Methane in the atmosphere
comes from many sources, including wetlands, rice cultivation,
termites, cows and other ruminants, forest fires, and fossil fuel
production. Some researchers have estimated that up to two
percent of atmospheric methane may originate through
dissociation of global methane hydrates.25 It has been shown
that methane is an important component of Earth’s carbon
cycle on geologic time scales. Whether methane once stored as
methane hydrate has contributed to past climate change or will
play a role in the future global climate remains unclear. A given
volume of methane causes 15 to 20 times more greenhouse gas
warming than carbon dioxide, so the release of large quantities
of methane could exacerbate warming and cause more
methane hydrate to destabilize. Extreme warming during the
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the Storegga31 and Cape Fear32 slides have addressed this topic
with inconclusive results. However, these studies suggest that
methane hydrate impact on seafloor stability in recent geologic
history may be small.
Continuous methane gas venting occurs in many marine
settings and may in some cases represent a naturally occurring
geohazard. Large concentrations of gas chimneys have been
documented in certain settings,33 which may cause widespread
gas releases or even sediment expulsion. Gas venting was initially
thought to have caused a collapse feature on the crest of Blake
Ridge, but later this feature was found to have resulted from
sedimentological processes.34 Compelling evidence for gas and
sediment expulsions was found in the “pingo-like features”
observed on the shallow Canadian Beaufort shelf.35 These
features appear to be a result of methane hydrate dissociation
related to melting of permafrost due to post ice-age sea level rise.
In comparison to most conventional hydrocarbon accumulations, methane hydrates occur at relatively shallow depths,
representing an operational geohazard to shallow drilling
and well completions. Mechanical disturbance and/or heating
of these shallow reservoirs through, for example, drilling or
emplacement of seafloor infrastructure such as pipelines can
cause the hydrates to dissociate, thus reducing the host sediment
strength and adversely impacting drilling operations and well
installations.
Results from several methane hydrate drilling programs,
including Ocean Drilling Program (ODP) Legs 164 and 204, and
more recently the Chevron-led Gulf of Mexico Joint Industry
Project (GOM-JIP) Legs I and II, Integrated Ocean Drilling
Program (IODP) Expedition 311, and National Gas Hydrate
Program (NGHP) Expedition 01 have shown that drilling
hazards associated with methane hydrate-bearing sections can be
managed through the careful control of drilling parameters.36,37
However, a longer-term and perhaps more difficult-to-constrain
risk is the potential for hydrate dissociation and sedimentwellbore instability caused by the heating of sediment around
production wells due to sustained flow of deeper, warmer
fluids.38,39
There is a significant lack of quantitative understanding of
operational geohazards because of the general lack of practical
field experience with methane hydrate systems. There is even a
greater lack of experience when dealing with operational
geohazards associated with the direct exploitation of methane
hydrates as a potential resource. More work is needed to understand the complete range of operational geohazards associated
with various types of methane hydrate occurrences in nature.
Paleocene−Eocene Thermal Maximum about 55 million years
ago may have been related to a large-scale release of global
methane hydrates.23 The impact of modern climate warming on
methane hydrate deposits does not appear to have led to
catastrophic breakdown of methane hydrates or major leakage of
methane to the ocean-atmosphere system from destabilized
hydrates. The vast majority of methane hydrates would require a
sustained warming over thousands of years to trigger
dissociation, and the released methane would encounter many
potential sinks before being emitted to the atmosphere.
However, methane hydrates, particularly those associated with
relict permafrost, may yet persist on arctic shelves that have been
inundated by post ice-age sea-level rise and are now dissociating
in response to longer-term climate processes.25
The controls on the inventory and flux of methane in marine
systems are poorly constrained; methane hydrate is a component
of a complex system, with inputs and outputs of methane over
time. Ultimately, methane generation is closely tied to the inputs
of organic carbon, although it is not yet clear how to best evaluate
the relationship between the amount and type of organic carbon
deposited on the seafloor and the quantity of methane generated.
In terms of outputs, it is important to quantify the amount of
methane lost from the system through naturally occurring gas
seeps and the amount consumed by anaerobic methane oxidation
in the sediments.
In addition to understanding the dynamics of carbon flux
associated with hydrate systems, strong interest exists in understanding how methane hydrate systems respond to natural and
anthropogenic perturbations. Dissociation of methane hydrate
due to warming or sea-level change can release methane into the
ocean system, affecting ocean chemistry (for example, “ocean
acidification”) and, potentially, climate and marine slope stability.
Past warming has been hypothesized to be responsible for global
methane hydrate dissociation events that have played a critical
role in climate change.23,25 However, the nature, mechanisms,
and extent of methane escape due to perturbations are poorly
understood. Moreover, the fate and extent to which methane
reaches the atmosphere is not well-constrained even in active
vents and seeps overlying modern methane hydrate systems.
These unknowns result in uncertainties in carbon cycle and
climate models.
2E. Methane Hydrates as Geohazards. Geohazards
associated with the occurrence of methane hydrates in nature
are generally classified as “naturally occurring” geohazards that
emerge wholly from geologic processes and “operational”
geohazards that may be triggered by human activity (Figure 6).26
As a geohazard, the dissociation of methane hydrate replaces
a rigid sediment component (i.e., methane hydrate) with free gas
and excess pore water, which may substantially reduce the
geomechanical stability of the affected sediment.
The two most important naturally occurring geohazards
associated with methane hydrate dissociation are slope instability
and wide-scale gas venting. The concept that methane hydrate
dissociation causes extensive slope instability has been around for
over three decades,27 and it has received further support with the
recognition that methane hydrates may play an important role in
the global carbon cycle.28 Several investigators have argued that
lowering global sea level and/or ocean warming establishes new
equilibrium conditions for marine methane hydrate stability,
which induces seafloor slope instability.29,30 However, evidence
has emerged that methane hydrate dissociation does not occur in
such a manner as to cause widespread slope instability. Field
investigations at the sites of major mass wasting events, such as
3. ADVANCEMENTS IN METHANE HYDRATE
MODELING, LABORATORY, AND FIELD
CHARACTERIZATION
To advance methane hydrate science and properly draw
generalized interpretations from site-specific field data requires
accurate laboratory and field data, and the development of
advanced laboratory and field measurement tools. These data
and tools are critical to the development of accurate and reliable
pore-scale and transport models, physical property and geochemical field and laboratory measurements, and reservoir
prediction models. The technical developments below describe
both routine and specialized needs for laboratory and field
measurements and modeling developments in the support of
methane hydrate research.
3A. Integrated Field and Laboratory Measurements
and Experiments. To assess the in situ nature of methane
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Figure 6. Geohazards associated with the occurrence of methane hydrate encompasses any condition that has the potential to negatively impact human
activity or the natural environment. In this review, “natural-occurring” geohazards refer to conditions associated with natural processes. “Operational”
geohazards are conditions triggered by human activities.26
hydrates requires the retrieval and analysis of hydrate-bearing
sediment cores that closely represent in situ conditions, which
has led to the development of advance hydrate pressure coring
systems. One of the first pressure core systems used to study
methane hydrates was the pressure core sampler (PCS). The
PCS is sealed in an autoclave core barrel that can withstand the
hydrostatic pressure at the coring depth and remains sealed as it
is brought to the surface. In the past, pressure core systems were
used to determine the in situ gas volume and composition of
recovered hydrate-bearing cores, and in some configurations, the
pressurized cores were analyzed using X-ray imaging systems.40
Significant advances have been made in the development
and implementation of pressure coring tools for hydrate drilling
expeditions.41,42 Systems include the Fugro Pressure Corer, which is
suitable for use in unlithified sediments, and the Fugro Rotary Pressure
Corer, which was designed to sample lithified sediment or rock. A new
generation of pressure coring systems has also been developed,
including the Pressure−Temperature Coring System (PTCS) and the
Hybrid Pressure−Coring System (Hybrid-PCS), which deliver longer
cores and feature robust ball-valve sealing systems.
Further developments have been made to enable the hydratebearing cores retrieved under pressure to be transferred and
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more advanced log-analysis models are required to characterize
methane hydrate in fractured (anisotropic) reservoir systems.48−50 New well logging tools designed to make directionally
oriented acoustic and propagation resistivity log measurements
provide the data needed to analyze the acoustic and electrical
anisotropic properties of highly interbedded and fracturedominated methane-hydrate reservoirs.
Advancements in nuclear magnetic resonance (NMR) logging
and wireline formation testing have also allowed for the characterization of methane hydrate at the pore scale. Integrated
NMR and formation testing studies from northern Canada and
Alaska have yielded valuable insight into how methane hydrates
are physically distributed in sediments and the occurrence
and nature of pore fluids (i.e., free water along with clay- and
capillary-bound water) in methane hydrate-bearing reservoirs.47
Information on the distribution of methane hydrate at the pore
scale has provided invaluable insight on the mechanisms
controlling the formation and occurrence of methane hydrate
in nature along with data on methane hydrate reservoir
properties (i.e., porosities and permeabilities) needed to
accurately predict gas production rates for various methane
hydrate production schemes.
It is necessary to continue to develop and widely implement
existing advanced field characterization tools to obtain reliable
and accurate field data on hydrate-bearing reservoirs during
predrilling, drilling, and postdrilling phases and production
programs. In particular, NMR and wireline formation testing tool
deployments are needed for detailed reservoir characterization.47
In addition, downhole tools such as cone penetrometers and in
situ formation pressure and temperature measurement devices
need to be included in future drilling expeditions to address the
outstanding methane hydrate related science challenges.
3C. Methane Hydrate Reservoir Modeling. Multiscale
models (pore-scale and reservoir-scale) development and
validation are needed to provide reliable assessment of the
methane hydrate resource potential, gas production rates, and
geomechanical/environmental impacts of methane hydrate
systems during production. Hydrate reservoir models that have
been developed include TOUGH+HYDRATE/TOUGH,
Fx/Hydrate (DOE-LBNL), CMG STARS (Computer Modeling
Group), HydrateResSim (DOE-NETL), MH-21 (National
Institute of Advanced Industrial Science and Technology,
Japan Oil Engineering Company, University of Tokyo),
HYDRES, and STOMP-HYD (PNNL, University of Alaska
Fairbanks). These reservoir model prediction tools have been
applied in numerous resource studies to estimate gas production
rates and reservoir response to depressurization, thermal
stimulation, and chemical injections.18,51,52
Simulating methane hydrate production requires solving a
complex combination of coupled heat, mass, and fluid-transport
equations, together with assessment of the formation and
dissociation of multiple solid phases in the reservoir. The
available simulation models use different approaches to solve the
problems, which can lead to discrepancies between some of the
models. Reservoir simulators are being further developed to
assess gas recoverability from hydrate-bearing sediments in
oceanic and Arctic environments using different production
methods and the geomechanical properties of hydrate-bearing
reservoir systems. To ensure the model predictions are accurate
and reliable, it is important to incorporate the correct physics
into the models, including accurate pore-scale, thermodynamic,
and transport information. The correct physics can only be
measured without depressurizing the recovered cores. These new
systems include the HYACINTH Pressure Core Analysis and
Transfer System (PCATS) that enables acoustic P-wave velocity,
gamma ray attenuation, and X-ray imaging of recovered pressure
cores all under in situ pressure conditions.40,41 Pressure Core
Characterization Tools (PCCT) have been developed43 that
include core manipulation tools and characterization chambers
to enable hydrological, thermal, chemical, biological, and
mechanical properties to be measured under pressure and
effective stress conditions.
Wider use of the existing pressure core sampling technologies
is needed to enable hydrate-bearing core retrieval at in situ
conditions from a more diverse range of geologic settings. These
pressurized cores need to be analyzed using physical property
laboratory measurements before, during, and after hydrate
production tests. Furthermore, pressure core technologies
should be advanced and developed to enable their implementation to be more robust and reliable and to include pressurized
triaxial mechanical testing capabilities.
The limited number of available pressure cores from natural
systems highlights the need for the synthesis of hydrate-bearing
cores in the laboratory. These synthetic hydrate-bearing cores are
critical to facilitate calibration and interpretation of valuable field
data by providing end members and reference samples. The
ability to synthesize hydrate-bearing sediments in the laboratory
also enables systematic, well-controlled, and well-defined studies
that cannot be performed with pressure core samples due to their
limited availability.
Significant progress has been made in developing synthesis
methods for methane hydrate formation in a range of hydratebearing sediment systems. These synthesis methods include:
(1) hydrate formation from dissolved gas, which leads to
heterogeneous nucleation and more uniform growth in the pore
space; (2) from partial water saturation, which results in
preferential formation at grain contacts; and (3) from hydrate
particles and ice seeds where the hydrate-bearing sediment
properties depend on the relative size of the hydrate particles and
sediment grains as well as hydrate saturation.44 To date, only a
few studies have been performed to characterize hydrate-bearing
sediment samples (both synthetic and a limited number of
natural core samples) using CT-X-ray imaging, porosity−
permeability petrophysical analysis, or acoustic measurements.
There have only been a limited number of laboratory studies to
investigate gas production from methane hydrate-bearing
sediments by depressurization, thermal stimulation,45 and CO2
injection.46 In summary, laboratory analyses of well-characterized synthetic hydrate-bearing sediment cores have been shown
to be an important source of information on occurrence and
behavior of methane hydrates in nature.
3B. Downhole Measurement Systems. Advanced downhole wireline and logging-while-drilling conveyed tools are now
routinely used to examine the petrophysical properties of
methane hydrate reservoirs and the distribution and concentration of methane hydrates within various reservoir systems.47
The most established and well-known use of well logs in methane
hydrate research are those that provide electrical resistivity and
acoustic velocity data (both compressional- and shear-wave data)
to estimate methane hydrate content (i.e., methane hydrate porevolume saturation) in various types of reservoirs. Recent
integrated sediment coring and well log studies have confirmed
that electrical resistivity and acoustic velocity data can yield
accurate methane hydrate saturations in sediment grainsupported (isotropic) systems such as sand reservoirs, but
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obtained by acquiring more laboratory and field characterization
data sets and by conducting more production tests.
Critical to the assessment of methane hydrate occurrence,
stability, and the resource production potential of methane
hydrate accumulations is the accurate determination of the
methane hydrate stability conditions. The thermodynamic phase
equilibria stability conditions for pure methane hydrate are
well-established for a wide range of moderate pressure and
temperature conditions.53 However, the thermodynamic stability
data and models are less established for methane hydrate-bearing
sediment systems, where the sediment system and pore-water
chemistry can have a significant impact on the phase equilibria
properties. When the pore-size distribution of the sediment
system is above around 55 nm, the methane hydrate phase
equilibria curves are indistinguishable from that of bulk methane
hydrate, without the presence of the porous media.54 Conversely,
smaller sediment pore-size distributions can result in a significant
increase in the dissociation pressure of methane hydrate at a
given temperature. For example, methane hydrate dissociation
pressures in 7 nm pores (of silica gel) were decreased by 20% to
100% compared to the corresponding dissociation pressures for
bulk methane hydrate.55 Similarly, as the pore-water chemistry
increases in complexity by containing different salt ions, there
will be a significant impact on the methane hydrate stability
conditions, with data and models for higher concentrations and
more complex salt systems being less reliable.
3D. Methane Hydrate Drill Site Review and Characterization. In recent years, there have been important advances in
the approaches and data used in predrilling site surveys. For
example, building on the results of Gulf of Mexico Joint Industry
Project (JIP) Leg I, a key objective of JIP Leg II drilling program
was to address the hypotheses that methane hydrate occurs in
sand reservoirs within the deepwater Gulf of Mexico and that
specific methane hydrate-in-sand accumulations can be identified
and characterized prior to drilling through an integrated
geophysical-geological prospecting approach.8
The site selection review process for JIP Leg II consisted
of simultaneously integrating advanced seismic inversion results
with the geological−geophysical analyses to evaluate the
presence of gas sources and sand-rich lithofacies linked by
migration pathways. JIP Leg II was launched with a total of
20 drill locations permitted within the Gulf of Mexico.8
Ultimately, the downhole logging data acquired during JIP Leg
II confirmed reservoir-quality sands within the methane hydrate
stability zone in all seven wells drilled during the expedition, with
methane hydrate occurrences closely matching predrill predictions in six of the wells. For most drilling projects, however,
not all of the data needed for a comprehensive predrill site survey
are readily available.
our understanding of these resources is still evolving. Research
coring and downhole logging operations carried out by the ODP,
IODP, government agencies, and several consortia have significantly improved our understanding of how methane hydrates
occur in nature. Government agencies in many countries are
interested in the energy resource potential of methane hydrates.
Countries including Japan, Korea, China, India, and the United
States have established effective national-led programs and
implemented ambitious methane hydrate research drilling and
testing programs. These mostly energy-focused research efforts,
along with a host of other research programs and field studies,
have also indicated that methane hydrates may be an important
part of the global carbon cycle and naturally destabilized methane
hydrates may be contributing to the buildup of atmospheric
methane. It is also possible that methane hydrates may represent
a geohazard under some conditions.
The scientific foundation has been built for the realization that
methane hydrates are a global phenomenon, occurring in
permafrost regions of the Arctic and in deepwater portions of
most continental margins worldwide. However, more work is
needed to create a better understanding of the impact of hydrates
on safety and seafloor stability as well as to provide data that can
be used by scientists in their study of climate change, geohazards,
and assessment of the feasibility of methane hydrates as a
potential future energy resource.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: tcollett@usgs.gov.
Present Address
G.M.: GE Oil & Gas, 17619 Aldine Westfield Rd, Houston,
Texas, 77073, United States.
Notes
The authors declare no competing financial interest.
Disclaimer: Any use of trade, product, or firm names is for
descriptive purposes only and does not imply endorsement by
the U.S. Government.
■
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